Alvin M. Weinberg, 91, who died of heart disease Oct. 18 at his home in Oak Ridge, Tenn., championed the promise and possibilities of nuclear energy while leading the Oak Ridge National Laboratory from an early era of weapons-building to later research into the effects of radiation on human genetics.

Dr. Weinberg felt strongly that nuclear energy would benefit society and urged the scientific establishment to focus on safety. He worked in the field since joining the Manhattan Project in 1942, became an adviser to presidents and was an early leader in investigations of greenhouse gases and alternative energy sources.

He also coined a number of terms to help explain scientific issues to ordinary people, including "big science," "technological fix" and "Faustian bargain."

His interest in improving the safety of nuclear reactors annoyed some nuclear power supporters and contributed to his being fired as the head of the Oak Ridge lab in 1972. But that did not dampen his belief in the future of nuclear power, nor in the necessity of commercial reactors.

The accident at Pennsylvania's Three Mile Island nuclear power plant in 1979 resulted in the release of low-level radiation, but a large-scale meltdown had been avoided, he noted, and he pointed to the need for better training and increased investment in nuclear power.

"To deny rebirth of nuclear energy is to deny human ingenuity and aspiration. This I cannot do," he said in 1995. "During my life I have witnessed extraordinary feats of human ingenuity. I believe that this struggling ingenuity will be equal to the task of creating the Second Nuclear Era."

Big science, the collaboration of large numbers of scientists, government officials, academics, administrators and contractors spending large sums of public money, has resulted in space exploration, oceanographic discoveries, the Human Genome Project and particle accelerators, but none of those accomplishments were achieved without trade-offs. Because society has a larger stake in big science projects, the work has to be acceptable to the public, Dr. Weinberg said. He developed criteria that help scientists and the public make those choices.

For example, while Dr. Weinberg was director of Oak Ridge, he was most interested in designing and building reactors, but public outcry caused him to realize that he should have paid more attention to the problem of nuclear waste.

To address that oversight, he proposed the creation of nuclear energy parks, where uranium could be mined, enriched, fabricated, used in reactors to generate electricity and reprocessed. The radioactive wastes could be buried or stored permanently.

"I still have this notion that nuclear energy will reemerge," he told an Oak Ridge interviewer in 1995. "Although we spent 50 years and an awful lot of money on the development of nuclear energy, I don't see the development as completed. If I had my druthers, I would say, 'Let's take out a clean sheet of paper, and let's design a new category of reactor that avoids all difficulties and is inherently safe.' "

Dr. Weinberg, son of Russian immigrants, was born and raised in Chicago, where he received his bachelor's, master's and doctoral degrees in physics from the University of Chicago. Three years after receiving his doctorate in 1939, he joined the University of Chicago group that developed the first nuclear chain reactor, and he helped produce the plutonium used for the bomb dropped on Nagasaki, Japan. At the time, he signed petitions urging that the bomb be exploded in an unpopulated area as a demonstration to wartime enemies, but "I've come to decide that dropping the bomb actually saved lives," he wrote in 1994.

After World War II ended, Dr. Weinberg was appointed research director at Oak Ridge and became the lab's top director in 1955. He suggested to Adm. Hyman Rickover that the Nautilus submarine be powered by a pressurized water reactor, which led to the nuclear Navy and the development of commercial nuclear power plants.

He co-wrote, with Nobel laureate Eugene Wigner, "The Physical Theory of Nuclear Chain Reactors," a standard text in the field. He also wrote two memoirs, "The First Nuclear Era: The Life and Times of a Nuclear Fixer" and "Reflections on Big Science." He was a member of the National Academy of Sciences and President Dwight D. Eisenhower's Science Advisory Committee, which called for better communication of scientific information to the public.

After leaving the Oak Ridge laboratory, he was named director of the U.S. Office of Energy Research and Development, which came up with the idea of a solar energy institute, now known as the National Renewable Energy Laboratory. Dr. Weinberg also chaired a federal commission that in 1977 recommended spending $100 million in the next decade to pinpoint the causes and effects of rising amounts of carbon dioxide in the atmosphere.

His first wife, Margaret Despres Weinberg, died in 1969. His second wife, Genevieve DePersio Weinberg, died in 2004.

Survivors include a son, Richard J. Weinberg of Durham, N.C.; a sister; and three grandchildren.